1. Field of the Invention.
The present invention relates to traffic light control systems, and, more particularly, to monolithic microwave and millimeter wave devices automobile traffic light control systems.
2. Description of the Related Art.
Traffic lights regulate roadway intersection traffic with a view to both safety and smoothness of vehicular flow. A typical traffic light control system for the intersection of a secondary street with a major thoroughfare includes a sensor in the pavement of the secondary street to detect the presence or speed of vehicles. Typically, the sensor includes a loop of wire imbedded in the pavement and detects the presence of a vehicle over the loop by the change of the inductance of the loop. The loop is part of a tuned circuit; so the presence of a vehicle is detected by the detuning of the circuit. The inductive loop detector is basically a presence detector, and to obtain other information such as speed and traffic spacing or to get early warning of an approaching vehicle, multiple loops in a single lane can be used. The use of presence detectors permits avoidance of cycling the traffic lights to green for the secondary street if no vehicles are waiting during the red. However, the sensor loops are expensive to install and to interface with the traffic light control box; and the loops must be replaced every three to five years with severe winter weather responsible for the shortest lifetimes.
Various radar and sonar traffic light control systems have been proposed. For example, U.S. Pat. No. 4,317,117 discloses a combination of Doppler radar and passive infrared sensor with the the Doppler radar powered by a discrete microwave tunnel diode feeding a waveguide and horn antenna through a probe coupler. The Doppler signal indicates movement and the infrared signal indicates presence. Similarly, the same inventor in U.S. Pat. No. 4,370,718 discloses a discrete tunnel diode, Gunn diode, Barritt diode or field effect transistor to generate microwave power and, optionally, also mix the returning Doppler shifted signal. Although the frequency of operation of the radar can be any frequency, higher frequencies are preferred to minimize the antenna size for a given beam width. The identification of individual wehicles requires that the beam width should not exceed approximately three degrees. Such beam widths are obtained with antennas 70 cm and 7 cm in diameter at 10 GHz and 100 GHz, respectively. Another consideration is that the radar signal frequency should be located in one of the high atmospheric attenuation windows in the frequency spectrum. Such high attenuation windows are centered at, for example, 60 GHz and 115 GHz.
Microwave and millimeter wave transmitters are usually designed around discrete components. A transmitter requires a power source (free-running or controlled oscillator) and a radiating element (antenna); however, the technologies typically used to fabricate the oscillator and the antenna are incompatible. Thus hybrid transmitters with the oscillator and the antenna on separate substrates is standard. Monolithic millimeter wave oscillators often use IMPATT diodes with the frequency of oscillation determined by circuit impedance and specific diode conditions or by injection-locking signals; and the oscillator power is typically coupled out by a microstrip transmission line and fed into a microstrip patch or waveguide antenna. See for example R. Dinger et al, A 10 GHz Space Power Combiner with Parasitic Injection Locking, 1986 IEEE MTT Symposium Digest 163 and K.Carver et al, Microstrip Antenna Technology, 29 IEEE Tr. Ant. Prop. 2 (1981). For a transmitter operating over a band of frequencies, the antenna and the oscillator are designed for close impedance match throughout the bandwidth of operation to minimize mismatch losses.
However, the known transmitters have the problems of mismatch losses, design difficulties, and hybrid fabrication cost and difficulties, and this translates into problems for traffic light control systems of low frequency operation and maintainence and replacement costs.